Such stirrers are known in a wide variety of configurations in the prior art and are used in different technical fields. For example, DE-A-25 11 717 discloses a stirrer having a motor-operated hollow shaft and a rotor fastened thereto. The rotor is provided with holes or additive outlet openings, via which an additive in the form of air passed through the hollow shaft can be introduced into a liquid to be treated in order to aerate it, as is required for example in the treatment of biological slurry. DE-U-93 06 907 describes a liquid manure aeration device having a rotor driven via a hollow shaft, wherein additives in the form of air or chemicals can be drawn in by the hollow shaft simply under the effect of the negative pressure generated by the rotor and can be admixed via a corresponding additive outlet opening provided in the lower region of the hollow shaft. However, in the case of the known stirrers, both the intermixing of the medium to be treated and the feeding of the additive to be admixed are capable of improvement.